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One-step synthesis of high quality kesteriteCu2ZnSnS4 nanocrystals – a hydrothermal approach

Vincent Tiing Tiong, John Bell and Hongxia Wang*

Full Research Paper Open Access

Address:School of Chemistry, Physics and Mechanical Engineering, Scienceand Engineering Faculty, Queensland University of Technology,Brisbane, QLD 4001, Australia

Email:Hongxia Wang* - hx.wang@qut.edu.au

* Corresponding author

Keywords:Cu2ZnSnS4 nanocrystals; formation mechanism; hydrothermal;thioglycolic acid

Beilstein J. Nanotechnol. 2014, 5, 438–446.doi:10.3762/bjnano.5.51

Received: 15 January 2014Accepted: 14 March 2014Published: 09 April 2014

This article is part of the Thematic Series "Nanostructures for sensors,electronics, energy and environment II".

Guest Editor: N. Motta

© 2014 Tiong et al; licensee Beilstein-Institut.License and terms: see end of document.

AbstractThe present work demonstrates a systematic approach for the synthesis of pure kesterite-phase Cu2ZnSnS4 (CZTS) nanocrystals

with a uniform size distribution by a one-step, thioglycolic acid (TGA)-assisted hydrothermal route. The formation mechanism and

the role of TGA in the formation of CZTS compound were thoroughly studied. It has been found that TGA interacted with Cu2+ to

form Cu+ at the initial reaction stage and controlled the crystal-growth of CZTS nanocrystals during the hydrothermal reaction. The

consequence of the reduction of Cu2+ to Cu+ led to the formation Cu2−xS nuclei, which acted as the crystal framework for the

formation of CZTS compound. CZTS was formed by the diffusion of Zn2+ and Sn4+ cations to the lattice of Cu2−xS during the

hydrothermal reaction. The as-synthesized CZTS nanocrystals exhibited strong light absorption over the range of wavelength

beyond 1000 nm. The band gap of the material was determined to be 1.51 eV, which is optimal for application in photoelectric

energy conversion device.

438

IntroductionThe development of new semiconductor light absorbing ma-

terials for applications in photovoltaic technologies is driven by

the necessity to overcome the key issues in the current PV tech-

nologies: the high production cost of silicon wafer used in the

first generation solar cells and the limited availability of raw

materials such as tellurium and indium used in CdTe and

Cu(Ga, In)Se2 (CIGS) based thin film solar cells, which has

raised significant concerns over their production scale [1]. In

the process of developing new PV materials that do not have

the above problems, the compound Cu2ZnSnS4 (CZTS) is

emerging as a promising new sustainable light absorbing ma-

terial for PV technologies. As a direct band p-type semicon-

ductor material, CZTS has a theoretical band gap of 1.5 eV and

has high light absorption coefficient (>104 cm−1) in the range of

Beilstein J. Nanotechnol. 2014, 5, 438–446.

439

visible and near infrared irradiation of solar spectrum [2-4].

Shockley–Queisser balanced calculations have predicted that

the theoretical efficiency of PVs using light absorbers like

CZTS is 32% [5].

It has been proposed that high-efficiency and low-cost photo-

voltaic devices can be made from CZTS nanocrystals [6,7].

This is due to the fact that thin film light absorber layers with

controlled thickness can be made from a slurry containing the

nanocrystals by a cost-effective method such as doctor blading,

spin coating and screen printing which can be scaled-up easily.

The recently reported thin film solar cells based on

Cu2ZnSn(S,Se)4 demonstrated a power conversion efficiency of

11.1%, which has approached the benchmark for large scale

production [8]. This great achievement shows the bright future

for CZTS based PVs. The highest efficiency CZTS solar cell

was made using hydrazine based sol–gel method. However,

hydrazine is a highly toxic, dangerously unstable solvent and

requires extra caution in handling and storage [9]. Therefore, a

safer, simple yet convenient method for fabrication of high

quality CZTS nanocrystals is desired.

The hydrothermal method has been widely used to synthesize

high quality nanocrystals with unique morphology and crystal

structure due to its advantage of simplicity of the procedure and

low production cost [10-17]. However, to the best of our knowl-

edge, the formation mechanism of CZTS in the hydrothermal

reaction has rarely been reported due to the complex reactions

involved in the system. Herein we report the synthesis of high

quality, pure kesterite phase, monodisperse CZTS nanocrystals

by a one-step hydrothermal procedure. Through thoroughly

investigating the factors that influence the morphology, crystal

size, and growth of CZTS nanocrystals, a mechanism that

depicts the formation process of CZTS compound is proposed.

It is found that the tiny amount of thioglycolic acid (TGA) used

in the precursor is crucial for the formation of pure kesterite

CZTS nanocrystals. The roles of TGA in the hydrothermal syn-

thesis are discussed.

ExperimentalMaterials: All the materials were provided by Sigma Aldrich

unless otherwise stated. Chemicals of copper(II) chloride dehy-

drate (CuCl2·2H2O), zinc chloride (ZnCl2) product of BDH,

tin(IV) chloride pentahydrate (SnCl4·5H2O), sodium sulfide

nonahydrate (Na2S·9H2O), thioglycolic acid (TGA) were all of

analytical grade and used as received without further purifica-

tion. Milli-Q water was used in this work.

Synthesis of CZTS nanocrystals by hydrothermal reaction:

In a typical experimental procedure, 0.2 mmol of CuCl2·2H2O,

0.1 mmol of ZnCl2, 0.1 mmol of SnCl4·5H2O, 0.5 mmol of

Na2S·9H2O and 18 μL of TGA were dissolved in 34 mL of

Milli-Q water under vigorous magnetic stirring. The solution

was then transferred to a Teflon-lined stainless steel autoclave

(Parr Instrument Company) of 45 mL capacity, which was then

sealed and maintained at 240 °C for 24 h. After that, the auto-

clave was allowed to cool to room temperature naturally. The

black precipitate was collected by centrifugation and washed

with deionised water and absolute ethanol for several times to

remove the ions in the end product. Finally, the product was

vacuum-dried at 60 °C for 5 h.

Characterisation: The crystallographic structure of the synthe-

sized samples was identified by X-ray diffraction (XRD,

PANanalytical XPert Pro Multi-Purpose Diffractometer (MPD),

Cu Kα, λ = 0.154056 nm). The room temperature Raman

spectra of the samples were recorded with a Raman spectrom-

eter (Renishaw inVia Raman microscope). The incident laser

light with the wavelength of 785 nm was employed as the exci-

tation source in micro-Raman measurement and the spectra

were collected by taking the average of 10 different spots. The

quantitative elemental analysis of the samples were character-

ized by field emission scanning electron microscopy (FESEM,

JEOL 7001F) at an acceleration voltage of 20.0 kV combined

with an energy dispersive X-ray spectroscopy (EDS). Transmis-

sion electron microscopy (TEM) images of the samples were

performed on a JEOL JEM-1400 microscope. High-resolution

TEM (HRTEM) and selected area electron diffraction (SAED)

images were obtained using JEOL JEM-2100 microscope at an

accelerating voltage of 200 kV. Ultraviolet–visible (UV–vis)

absorption spectrum of the sample was measured at room

temperature using a Varian Cary 50 spectrometer. The chem-

ical state of each element in the samples was determined using

Kratos Axis ULTRA X-ray photoelectron spectrometer (XPS).

Results and DiscussionSynthesis of CZTS nanocrystalsThe XRD pattern of the CZTS nanocrystals prepared at 240 °C

for 24 h using 18 μL of TGA in the hydrothermal reaction is

shown in Figure 1a. All the XRD diffraction peaks can be well

indexed to the corresponding crystal planes of kesterite CZTS

(JCPDS 01-75-4122) [5,18]. The Raman spectrum of the

hydrothermal product is shown in Figure 1b. The strong peak at

336 cm−1 together with two shoulder peaks at 288 and

372 cm−1 further confirm the formation of CZTS [19]. No other

characteristic peaks corresponding to impurities such as Cu2−xS

(475 cm−1), SnS2 (315 cm−1), ZnS (278 and 351 cm−1),

Cu2SnS3 (297 and 337 cm−1), and Cu3SnS4 (318 cm−1) that

might form in the hydrothermal reaction are observed,

suggesting the highly purity of the synthesized CZTS material

[19,20]. The morphology and particle size of the CZTS

nanocrystals are shown in Figure 1c, which suggests the CZTS

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440

Figure 1: (a) XRD patterns, (b) Raman spectra, (c) TEM image, (d) HRTEM image and (e) SAED pattern of CZTS nanocrystals hydrothermallysynthesized at 240 °C for 24 h.

nanocrystals are monodisperse with crystal sizes around

10 ± 3 nm. High-resolution TEM (HRTEM) image in Figure 1d

illustrates the crystal interplanar spacing of 3.12 Å, which can

be ascribed to the (112) plane of kesterite phase CZTS. The

diffraction spots in the selected area electron diffraction

(SAED) pattern illustrated in Figure 1e can all be indexed to the

(112), (220), (224) and (420) planes of kesterite CZTS respect-

ively, further confirming the phase purity of the material. The

atomic ratio of Cu/Zn/Sn/S in the material is 1.97/1.04/1.03/

3.96 according to energy dispersive X-ray spectroscopy (EDS)

results (see Table 1), which is consistent with the stoichio-

metric value of 2/1/1/4 of CZTS (by considering the experi-

mental error of EDS detector).

X-ray photoelectron spectrometry (XPS) measurement was

conducted to monitor the valence states of all four elements in

the as-synthesized CZTS nanocrystals. Figure 2 displays the

high resolution XPS analysis for the four constituent elements:

Cu 2p, Zn 2p, Sn 3d and S 2p of CZTS nanocrystals. The spec-

trum of Cu 2p shows two peaks at 932.14 and 951.99 eV with a

splitting of 19.85 eV, which is in good agreement with the stan-

dard separation (19.9 eV) of Cu(I). The peaks of Zn 2p appear

at 1022.29 and 1045.46 eV with a split orbit of 23.17 eV, which

can be assigned to Zn(II). The peaks of Sn 3d show binding

energies at 486.35 and at 494.77 eV respectively, which is in

good agreement with the value of Sn(IV). The S 2p peaks are

located at 161.76 and 162.92 eV, which are consistent with the

binding energy of sulfur in sulfide state of CZTS. These results

are in agreement with the reported values of the binding state of

the elements of CZTS [18,21].

Influence of different reaction conditionDifferent reaction conditions such as reaction temperature, reac-

tion duration and concentration of capping agent have been

reported to have significant impacts on the morphology, particle

size as well as the optical properties of the materials formed in a

hydrothermal reaction [17,22]. Hence, a series of experiments

under different reaction conditions were carried out to under-

stand the role of TGA and to gain in-depth insight into the for-

mation mechanism of CZTS nanocrystals.

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441

Figure 2: XPS spectra of CZTS nanocrystals synthesized at 240 °C for 24 h.

Figure 3: (a) XRD patterns and (b) Raman spectra of the hydrothermal products synthesized with different TGA concentration in the precursor solu-tion.

Influence of TGA concentrationTGA has been widely used in hydrothermal synthesis of metal

sulfide such as ZnS, SnS etc. It has been reported that the

content of TGA influences the morphology of the hydrothermal

product [23,24]. The effect of the content of TGA on the forma-

tion of CZTS compound was investigated in this work. The

XRD results of the hydrothermal products synthesized with

three different TGA concentrations are shown in Figure 3a. As

can be see, when there is no TGA, the XRD pattern of

hydrothermal product contains the peaks corresponding to

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442

Table 1: Quantitative elemental analysis of CZTS nanocrystals synthesized with different TGA content in the precursor solution of the hydrothermalreaction.

Ratio 0 μL 18 μL 180 μL

Cu/Zn/Sn/S 2.02/0.84/1.18/3.96 1.97/1.04/1.03/3.96 1.96/1.02/0.93/4.09[Cu]/([Zn+Sn]) 1/1.00 1/1.05 1/0.99

[Zn]/[Sn] 1/1.40 1/0.99 1/0.91

Figure 4: TEM images of CZTS nanocrystals synthesized using (a) 0, (b) 18 and (c) 180 μL of TGA at 240 °C for 24 h.

kesterite CZTS and three other peaks that are attributed to SnO2

(JCPDS 00-001-0625) impurity. Raman spectra in Figure 3b

indicates that, in the absence of TGA, a weak peak located at

298 cm−1 which is assigned to ternary Cu2SnS3, is detected

along with the CZTS peaks. However, the peaks corresponding

to impurities disappear when tiny amount of TGA (18 μL) is

added to the precursor solution. Further increase the content of

TGA does not influence the XRD and Raman results of the ma-

terial. Thus, the very small amount of TGA in the hydrothermal

precursor solution determines the compositional purity of the

synthesized CZTS material.

The quantitative elemental analysis of these three CZTS

samples (Table 1) shows that the content of Sn element in the

hydrothermal sample is very high ([Zn]/[Sn] = 1/1.40) without

TGA. And the ratio of Cu/(Zn+Sn) and [Zn]/[Sn] is close to 1

when adding only 18 μL TGA in the hydrothermal reaction

system. The Sn rich and Zn poor composition in the sample

with no TGA is probably due to the formation of Cu2SnS3 and

SnO2 impurity as confirmed by the above XRD and Raman

spectrum. The EDS analysis also shows that the elemental com-

position of the CZTS material using excessive amount of TGA

(180 μL) leads to the slightly reduced Sn content relative to Zn.

The morphology of the synthesized CZTS nanocrystals

prepared with the three different amounts of TGA measured by

TEM is shown in Figure 4. It shows that when there is no TGA

in the precursor solution, agglomerates with irregular shape

with size ranging from 10–150 nm are obtained. When 18 μL

TGA is used in the reaction system, uniform and monodisperse

CZTS nanocrystals with an average size of 10 nm are obtained.

With increasing the TGA concentration to 180 μL, the size

distribution of the CZTS nanocrystals becomes less uniform and

some triangular-like shape nanocrystals are observed. The

above results demonstrate that high concentration of TGA is not

favourable for the formation of monodisperse CZTS nanocrys-

tals. At a high concentration, TGA might form a colloid which

wraps a certain surface of CZTS particles, inhibiting the growth

of crystals in all directions [25]. Hence, it is rational to conjec-

ture that TGA might play two key roles in this work. One is to

prevent aggregation of CZTS nanocrystals by capping on the

generated nanocrystals to reduce the surface energy (steric

hindrance) during the hydrothermal process; the other role is

selective adsorption on certain facets of CZTS nanocrystals and

kinetic control of the growth rates of these facets [4,25].

Influence of reaction durationFigure 5b,c shows the XRD patterns and Raman spectra of the

CZTS nanocrystals synthesized at different hydrothermal reac-

tion duration (from 0.5 h to 24 h). The XRD pattern of the

precipitate collected from the hydrothermal precursor solution

prior to the reaction is shown in Figure 5a. The result suggests

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Table 2: Quantitative elemental analysis of CZTS nanocrystals synthesized at different reaction duration.

Ratio 0 h 0.5 h 2 h 8 h 24 h

Cu/Zn/Sn/S 5.0/0.1/0.1/2.8 2.1/0.9/0.8/4.1 2.1/1.0/0.9/4.0 2.1/1.0/1.0/3.9 2.0/1.0/1.0/4.0[Cu]/([Zn+Sn]) 1/0.04 1/0.82 1/0.90 1/0.97 1/1.05

[Zn]/[Sn] — 1/0.93 1/0.92 1/0.96 1/0.99

Figure 5: (a) XRD of the precipitate collected from the precursorsolution prior to hydrothermal reaction and (b) XRD patterns and(c) Raman spectra of the samples synthesized at different reactionduration.

that Cu7S4 (JCPDS 23-0958) and Cu1.8S (JCPDS 56-1256) are

formed immediately in the precursor solution prior to the

hydrothermal reaction. When the hydrothermal reaction is

proceeded for only 0.5 h (Figure 5b), three XRD peaks that can

be assigned to kesterite CZTS are observed. The intensity of the

diffraction peaks increases gradually with the increase of the

reaction time. A characteristic peak located at around 32.9°

corresponding to the (200) plane of kesterite CZTS is notice-

able only when the reaction duration is extended to 8 h and

beyond. The gradual increment of the intensity of diffraction

peaks suggests the improvement of crystallinity of the CZTS

nanocrystals. However, the Raman spectra (Figure 5c) of the

hydrothermal products synthesized at different reaction dura-

tion show that, at a shorter reaction time (less than 8 h), the

hydrothermal products contain a mixture of CZTS, Cu2SnS3

and Cu3SnS4. Pure phase CZTS nanocrystals are only obtained

at reaction duration of 24 h. The Raman spectra also show that,

the intensity of CZTS peak at 337 cm−1 increases and the peak

at 327 cm−1 which belongs to Cu3SnS4 decreases as the reac-

tion time is prolonged [26]. The red shift of the peak at 298 to

287 cm−1 denotes the complete transformation of Cu2SnS3 to

CZTS at 24 h hydrothermal reaction. Since no peak corres-

ponding to ZnS is observed in the entire Raman spectra, it

suggests that CZTS compound in the hydrothermal reaction

might be formed through diffusion of Zn ion to the ternary

CuxSnSy (x = 2,3, y = 3,4) compound.

The atomic ratios of [Cu]/([Zn]+[Sn]) and [Zn]/[Sn] of the

synthesized hydrothermal products obtained at different reac-

tion duration are shown in Table 2. The much higher content of

copper in the sample obtained prior to the hydrothermal reac-

tion and the nearly two-fold of Cu relative to S is consistent

with the observation of Cu2−xS product by the XRD measure-

ment as discussed above. As the reaction time is prolonged

from 0.5 h to 24 h, the [Cu]/([Zn]+[Sn]) ratio is reduced from 1/

0.82 to 1/1.05, while the [Zn]/[Sn] ratio shows a little change

from 1/0.93 to 1/0.99. The nearly stoichiometric composition

(1.97/1.04/1.03/3.96) of CZTS is obtained at a reaction time of

24 h. Since no ZnS is detected in the samples synthesized at 0.5

and 2 h, we believe that the content of Cu2SnS3 impurity in

these hydrothermal products is very low. The high content of

Zn and Sn compound at 0.5 h and 2 h also suggests that CZTS

compound is formed rapidly in the hydrothermal reaction.

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Figure 6: TEM images of (a, b, c) Cu7S4 and Cu1.8S nanocrystals collected prior to hydrothermal reaction and CZTS nanocrystals synthesized atdifferent reaction duration: (d, e) 0.5 h, (f) 2 h, (g) 8 h, and (h) 24 h respectively.

The TEM images of the hydrothermal products collected at

different reaction duration are shown in Figure 6. Figure 6a

illustrates that, prior to the hydrothermal process, the precipi-

tate obtained from the precursor solution is consisting of

microspheres with size around 20–250 nm. The HRTEM indi-

cates that the microparticle is the result of aggregation of

numerous oval-like nanocrystals with size ranging from

10–30 nm. The HRTEM image of the material (inset of

Figure 6b) shows the lattice fringe of a nanocrystal with an

interplanar spacing of 1.87 Å, which is in good agreement with

the (886) plane of monoclinic structure of Cu7S4. Besides that,

the lattice fringe of nanocrystal with an interplanar spacing of

1.97 Å which can be ascribed to (220) plane of cubic structure

of Cu1.8S was also found as illustrated in the inset of Figure 6c.

These finding are consisting with the above shown XRD

pattern. Figure 6d and 6e show that when the hydrothermal

reaction is proceeded for 0.5 h, the dominant products are

nanocrystals with irregular size of about 2–5 nm. The inter-

planar spacing of the crystals is 3.125 Å, which belongs to the

(112) plane of CZTS. This result further confirms the rapid for-

mation of CZTS compound in the hydrothermal reaction.

Figure 6f shows that after hydrothermal reaction for 2 h, the

agglomeration starts to break into small particles with size

ranging from 3–20 nm. As the reaction time is prolonged to 8 h

(Figure 6g), nanocrystals with size in the range of 5–10 nm

appear in the product, and meanwhile the large agglomerates

disappear. Upon gradual evolution of the CZTS nanostructures,

nanoparticles with uniform distribution are obtained after reac-

tion duration of 24 h (Figure 6h).

Formation mechanismIt is normally assumed that metal cations in a hydrothermal

reaction are firstly associated with TGA in the precursor solu-

tion to form metal-TGA complexes prior to the hydrothermal

reaction [22,23]. However, the formation of Cu7S4 and Cu1.8S

compounds in our case suggests that Cu2+ is reduced to Cu+ by

interaction with the –SH (thiol) group of TGA (oxidation of

TGA to dithiodiglycolate) [27]. The XRD pattern of the precipi-

tate collected from the precursor solution without TGA prior to

hydrothermal reaction reveals that CuS (JCPDS 6-0464) instead

of Cu2−xS (x = 0–0.2) is formed (Figure 7). This confirms the

reduction role played by TGA in the hydrothermal reaction

system.

We believe that the initially formed Cu2−xS nanocrystals in the

precursor solution prior to hydrothermal reaction act as nuclei

for the formation of kesterite CZTS. The formed Cu7S4 and

Cu1.8S material has a monoclinic and cubic crystal structure,

respectively as confirmed by XRD measurement shown in

Figure 5a. At relatively high reaction temperature, the copper

ions in Cu2−xS have a relatively high mobility which can

accommodate an exchange with other metal ions at a low

energy cost [28]. In addition, the crystal structure of Cu1.8S has

a cubic close packing (ccp) array of sulfur ions which is similar

to the arrangement of sulfur in kesterite CZTS crystal frame-

work. Hence, the interdiffusion of cations such as Zn2+ and

Sn4+ to Cu2−xS crystal to form CZTS compound is feasible

because there is little lattice distortion in such process [29].

Thus, the typical reaction condition (220 °C and above) is

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Figure 8: Schematic illustrations of the formation process for kesterite CZTS nanoparticles.

Figure 7: XRD pattern of the precipitate collected from the precursorsolution without TGA prior to hydrothermal reaction.

believed to facilitate the chemical transformation from Cu2−xS

to CZTS. Since no other binary products such as SnS2 and ZnS

are discovered in the entire hydrothermal process, and only

Cu2SnS3 and Cu3SnS4 are detected in the Raman spectra, we

believe Sn4+ cations are firstly incorporated into the crystal

lattice of Cu2−xS and replaced parts of Cu+ ion, followed by the

rapid doping of Zn2+ to form CZTS compound in the

hydrothermal process. The formation of CZTS compound as

confirmed by TEM at the very short reaction time (0.5 h)

suggests the fast diffusion rate of Zn and Sn ions to the lattice

of Cu2−xS nuclei crystals in the hydrothermal process. More-

over, when the reaction duration is extended to 8 h, the Zn/Sn

ratio is increased to 1/1.04 which matches well with the theo-

retical value of 1:1 in CZTS. With the proceeding of the reac-

tion, the primary CZTS crystal nucleus grows to nanoparticles

with different sizes. Based on Ostwald ripening process, the

small crystal nucleus will grow up to form larger crystals

because the large one has lower surface free energy [30]. TGA

molecules which are adsorbed on the nanocrystals surface may

restrict the growth of CZTS crystals and slow down the growth

process, leading to the formation of monodisperse CZTS

nanocrystals. Based on the above analysis, a schematics

showing the formation mechanism for CZTS compound in the

hydrothermal reaction is shown in Figure 8.

The UV–visible spectra of the hydrothermal samples synthe-

sized at different reaction time are shown in Figure 9. It is

found that the onset for light absorption of the material gradu-

ally shifts to longer wavelengths with the elongation of the reac-

tion duration. The calculation of the band gap of the materials

which is determined by extrapolation of the plot of (Ahν)2 vs hν

(Inset of Figure 9) where A = absorbance, h = Planck’s constant,

and ν = frequency, shows that the hydrothermal products at

reaction time of 0.5, 2, 8, and 24 h have band gap of 1.92, 1.76,

1.63 and 1.51 eV respectively. The decrement of the band gap

value is due to the improvement of CZTS purity because impu-

rities such as Cu2SnS3 and Cu3SnS4 have larger band gap than

CZTS [31,32].

Figure 9: UV–visible absorption spectra of the CZTS nanocrystalssynthesized at different reaction duration and the inset image showsthe (Ahν)2 vs hν plots with corresponding fitting of the samples.

ConclusionHigh quality, pure kesterite phase CZTS nanocrystals with

uniform size distribution have been successfully synthesized by

a facile one-step hydrothermal route based on a precursor solu-

tion containing thioglycolic acid (TGA) as surfactant. The role

of TGA in the hydrothermal reaction is clarified and a forma-

tion mechanism of CZTS compound in the hydrothermal reac-

tion is proposed. It is believed that the formation of CZTS is

Beilstein J. Nanotechnol. 2014, 5, 438–446.

446

initiated by the formation of Cu2−xS nanocrystals as a result of

reduction of Cu2+ by TGA to become Cu+. This is followed by

the rapid diffusion of cations Sn4+ and Zn2+ to the crystal

framework of Cu2−xS to form CZTS. The good optical prop-

erties and suitable band gap of 1.51 eV of the synthesized CZTS

nanocrystals indicate the promise of this material for applica-

tion in low cost thin film solar cells.

AcknowledgementsThe authors appreciate the technical assistance by Dr. Barry

Wood from University of Queensland for the XPS measure-

ments. This work was funded by the Vice-Chancellor Fellow-

ship Scheme of Queensland University of Technology and

Australian Research Council (ARC) Future Fellowship

(FT120100674), Australia.

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